So why would the telephone be dead? Bridge your device across the line. Its idle DC resistance must be equal or greater than 15K and the 20Hz impedance should be on the order of 7 to 14K. If there is only one other ringer across the line, you won’t affect the network.
One of the niftiest ways to detect the presence of ringing is with a NE2 lamp, diode, capacitor and resister voltage divider. It forms ½ wave rectifier that can turn on a CMOS gate. Select a RC time constant long enough to ensure that voltage spikes won’t accidently trip it. The minimum must ignore time is 25ms. Once ringing is detected, close the loop with a relay and your DTMF detector. Note that the impedance of your circuit should be either 600 or 900 ohms depending on if you intend to use it on a trunk or line but the receiver is specified to work into a 600 ohm source. In your case, I would guess the correct answer would be 900 ohms to the network and 600 ohms to the receiver. Also, it should be designed to pull a minimum off hook current of between 20ma and 35ma. The CO must drop the call if the current drops below 18ma and will have a feed resistance of 200 ohms plus loop resistance. Loop current will not normally exceed 35ma but could go as high as 50ma if you live next door to the CO or if they are using loop extenders. If the latter is the case, the maximum DC voltage could be as high as 130 volts so caution is advised.
If you have never done it before, DTMF can be picky about what is a good set of tones and what should be rejected.
• Both frequencies must fall within 1.5% of their nominal values to be detected and must be rejected if either is more than 3.5% from nominal.
• Must reject tone pairs that have a duration of less than 23ms and record ones with a duration of 40ms or longer. It must also keep up with a cycle time of 93ms or longer.
It goes on from there to get picky about tone levels as far as absolute levels, twist (the difference between the levels of the two tones) and about a dozen other factors. All the details are spelled out in the EIA/ANSI/TIA-464-C section 7.1.5.
Last, if you intend to use this on the public network, you need to apply to the FCC for type acceptance of your device.